Bacterial infections remain among the most common and deadly causes of human disease. Infectious diseases are the third leading cause of death in the United States and the leading cause of death worldwide (Binder et al., Science 284:1311-1313 (1999)). Multi-drug-resistant bacteria now cause infections that pose a grave and growing threat to public health. It has been shown that bacterial pathogens can acquire resistance to first-line and even second-line antibiotics. (See, Stuart B. Levy, The Challenge of Antibiotic Resistance, in Scientific American, 46-53 (March 1998); Walsh, C. (2000) Nature 406, 775-781; Schluger, N. (2000) Int. J. Tuberculosis Lung Disease 4, S71-S75; Raviglione et al., (2001) Ann. NY Acad. Sci. 953, 88-97). New approaches to drug development are necessary to combat the ever-increasing number of antibiotic-resistant pathogens.
The present invention provides one such approach, which involves the transcription machinery of bacteria. RNA is synthesized in cellular organisms by a complex molecular machine, known as RNA polymerase (“RNAP”). In its simplest bacterial form, RNAP comprises at least four subunits with a total molecular mass of around 400 kDa. RNAP mediates the transcription of DNA to produce RNA. Bacterial RNAP is a multimeric protein consisting of subunits α2, β, β′, and ω. An σ factor is required for initiation of transcription by forming a holoenzyme complex.
Transcription involves the following steps (Record et al. 1996): (i) RNAP binds to promoter DNA, to yield an RNAP-promoter closed complex; (ii) RNAP melts ˜14 bp of promoter DNA surrounding the transcription start site, to yield an RNAP-promoter open complex; (iii) RNAP begins synthesis of RNA, typically carrying out multiple rounds of abortive initiation (synthesis and release of RNA products <9-11 nt in length), as an RNAP-promoter initial transcribing complex; and (iv), upon synthesis of an RNA product of a critical threshold length of 9-11 nt, RNAP breaks its interactions with promoter DNA and begins to translocate along DNA, processively synthesizing RNA as an RNAP-DNA elongation complex.
Currently, there are a few known antibiotics that target RNAP, most notably, rifampicin and rifampicin analogs (See Mitchison, D. (2000) Int. J. Tuberculosis Lung Disease 4, 796-806). Rifampicin is the only anti-tuberculosis compound able to rapidly clear infection and prevent relapse. Without rifampicin, treatment lengths must increase from 6 months to at least 18 months to ensure prevention of relapse. Rifampicin acts by specifically inhibiting RNAP (Campbell et al., (2001) Cell 104, 901-912). Rifampicin binds to a site adjacent to the active center of bacterial RNAP, the exit channel, and physically prevents synthesis of products longer than ˜4 nucleotides. Unfortunately, tuberculosis strains resistant to rifampicin (and rifampicin analogs) are becoming widespread, effectively removing rifampicin from the therapeutic arsenal. There is a need for novel antibiotics that target the same bacterial enzyme as rifampicin, namely RNAP (and thus that have the same biochemical and therapeutic effects as rifampicin). There is also a need to develop methods for identifying antibiotics that interfere with bacterial RNAP.
Recently crystallographic structures have been determined for bacterial RNAP and eukaryotic RNAP II, and, based on the crystallographic structures, biophysical results, and biochemical results, structural models have been proposed for transcription initiation and elongation complexes (Zhang et al., (1999) Cell 98, 811-824; Cramer et al., (2000), Science 288, 640-649; Naryshkin et al., (2000) Cell 101, 601-611; Kim et al., (2000) Science 288, 1418-1421; Korzheva et al., (2000) Science 289, 619-625; Ebright, R. (2000) J. Mol. Biol. 304, 687-689; Cramer et al., (2001) Science 292, 1863-1876; Gnatt et al., (2001) Science 292, 1876-1882; Mekler et al., (2002) Cell 108, 599-614; Murakami et al., (2002) Science 296, 1280-1284; Murakami et al., (2002) Science 296, 1285-1290; Vassylyev et al., (2002) Nature 417, 712-719; Bushnell et al., (2004) Science 303, 983-988; Westover et al., (2004) Science 303, 1014-1016). The structural models include an approximately 30 Å long, 15 Å wide channel, known as the “RNA-exit-channel,” that connects the RNAP active-center cleft to the RNAP exterior. In transcription initiation complexes, transcription initiation factors occupy this channel: i.e., initiation factor σ region 3.2 (also known as the “σR3/σR4 linker” or “σ3/σ4 linker”) in the case of bacterial transcription initiation complexes and transcription initiation factor IIB N-terminal domain in the case of eukaryotic RNAP II transcription initiation complexes. In transcription elongation complexes, the nascent RNA product occupies this channel.